The addition of Ti or Ta to Nb3Sn is required to obtain the highest Bc2 (upper critical field) and highest critical currents at high (˜>12T) magnetic fields. (Nb, Ti)3Sn has lower AC losses than the alternative (Nb, Ta)3Sn material, therefore making it an appealing source for superconducting magnet makers concerned with rapidly changing magnetic fields. (Nb, Ti)3Sn also has better strain tolerance than (Nb, Ta)3Sn making it more useful in high stress magnet designs. (Nb, Ta)3Sn composites are commonly made with Nb7.5 wt % Ta filaments, which are harder than the pure Nb filaments used in (Nb, Ti)3Sn composites and thus more difficult to fabricate.
The present most common method for adding Ti to internal tin wire is by use of a Sn—Ti alloy. During heat treatment, the Ti in the Sn diffuses with the Sn through the Cu matrix of the composite wire to react with the Nb filaments and form the necessary high Bc2 phase. However several problems arise in use of this conventional Sn—Ti alloy approach. One problem is that hard, rod shaped Sn—Ti intermetallic particles of dimensions ˜25 μm diameter by ˜100 μm long are unavoidably formed during the casting of the Sn—Ti alloy. These intermetallic particles break filaments (and eventually the wire) during fabrication when their dimension becomes comparable with the tin reservoir dimension. The consequence is that the minimum diameter of the wire or the elements used in the wire is limited. Furthermore the Sn—Ti alloy is difficult and expensive to produce. Quality control is also an issue, which is especially problematic in light of the volumes of alloy that would be required to fulfill the wire orders for next generation fusion magnets.
Another method of Ti addition to internal tin or bronze process wire is to use a Ti or Nb—Ti rod in the center of each of the Nb filaments, so that the Ti diffuses from the center of each filament during heat treatment. The problem with this method is that it indeed requires every filament to have a Ti or Nb—Ti source in its center, which is prohibitively expensive.
Yet another method of Ti addition to bronze process wire is to use a Cu—Sn—Ti alloy matrix instead of Cu—Sn. The problem with this method is that is difficult to fabricate without cracking.